Lattice matchable alloy for solar cells

ABSTRACT

An alloy composition for a subcell of a solar cell is provided that has a bandgap of at least 0.9 eV, namely, Ga 1-x In x N y As 1-y-z Sb z  with a low antimony (Sb) content and with enhanced indium (In) content, and enhanced nitrogen (N) content, achieving substantial lattice matching to GaAs and Ge substrates and providing both high short circuit currents and high open circuit voltages in GaInNAsSb subcells for multijunction solar cells. The composition ranges for Ga 1-x In x N y As 1-y-z Sb z  are 0.07≦x≦0.18, 0.025≦y≦0.04 and 0.001≦z≦0.03.

CROSS-REFERENCES TO RELATED APPLICATIONS

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STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

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BACKGROUND OF THE INVENTION

The present invention relates to multifunction solar cells, and inparticular to high efficiency solar cells comprised of III-Vsemiconductor alloys.

Multifunction solar cells made primarily of III-V semiconductor alloysare known to produce solar cell efficiencies exceeding efficiencies ofother types of photovoltaic materials. Such alloys are combinations ofelements drawn from columns III and V of the standard Periodic Table,identified hereinafter by their standard chemical symbols, names andabbreviation. (Those of skill in the art can identify their class ofsemiconductor properties by class without specific reference to theircolumn.) The high efficiencies of these solar cells make them attractivefor terrestrial concentrating photovoltaic systems and systems designedto operate in outer space. Multifunction solar cells with efficienciesabove 40% under concentrations equivalent to several hundred suns havebeen reported. The known highest efficiency devices have three subcellswith each subcell consisting of a functional p-n junction and otherlayers, such as front and back surface field layers. These subcells areconnected through tunnel junctions, and the dominant layers are eitherlattice matched to the underlying substrate or are grown overmetamorphic layers. Lattice-matched devices and designs are desirablebecause they have proven reliability and because they use lesssemiconductor material than metamorphic solar cells, which requirerelatively thick buffer layers to accommodate differences in the latticeconstants of the various materials. As set forth more fully in U.S.patent application Ser. No. 12/217,818, entitled “GaInNAsSb Solar CellsGrown by Molecular Beam Epitaxy,” which application is incorporatedherein by reference, a layer made of GaInNAsSb material to create athird junction having a band gap of approximately 1.0 eV offers apromising approach to improving the efficiency of multijunction cells.Improvements are nevertheless to be considered on the cell described inthat application.

The known highest efficiency, lattice-matched solar cells typicallyinclude a monolithic stack of three functional p-n junctions, orsubcells, grown epitaxially on a germanium (Ge) substrate. The topsubcell has been made of (Al)GaInP, the middle one of (In)GaAs, and thebottom junction included the Ge substrate. (The foregoing nomenclaturefor a III-V alloy, wherein a constituent element is shownparenthetically, denotes a condition of variability in which thatparticular element can be zero.) This structure is not optimal forefficiency, in that the bottom junction can generate roughly twice theshort circuit current of the upper two junctions, as reported by J. F.Geisz et al., “Inverted GaInP/(In)GaAs/InGaAs triple-junction solarcells with low-stress metamorphic bottom junctions,” Proceedings of the33rd IEEE PVSC Photovoltaics Specialists Conference, 2008. This extracurrent capability is wasted, since the net current must be uniformthrough the entire stack, a design feature known as current matching.

In the disclosure of above noted U.S. patent application Ser. No.12/217,818, it was shown that a material that is substantially latticematched to Ge or GaAs with a band gap near 1.0 eV might be used tocreate a triple junction solar cell with efficiencies higher than thestructure described above by replacing the bottom Ge junction with ajunction made of a different material that produces a higher voltage.

In addition, it has been suggested that the use of this 1 eV materialmight be considered as a fourth junction to take advantage of the entireportion of the spectrum lying between 0.7 eV (the band gap forgermanium) and 1.1 eV (the upper end of the range of bandgaps for the ˜1eV layer). See for example, S. R. Kurtz, D. Myers, and J. M. Olson,“Projected Performance of Three and Four-Junction Devices Using GaAs andGaInP,” 26th IEEE Photovoltaics Specialists Conference, 1997, pp.875-878. Ga_(1-x)In_(x)N_(y)As_(1-y) has been identified as such a 1 eVmaterial, but currents high enough to match the other subcells have notbeen achieved, see, e.g., A. J. Ptak et al., Journal of Applied Physics98 (2005) 094501. This has been attributed to low minority carrierdiffusion lengths that prevent effective photocarrier collection. Solarsubcell design composed of gallium, indium, nitrogen, arsenic andvarious concentrations of antimony (GaInNAsSb) has been investigatedwith the reported outcome that antimony is helpful in decreasing surfaceroughness and allowing growth at higher substrate temperatures whereannealing is not necessary, but the investigators reported thatantimony, even in small concentrations is critical to be avoided asdetrimental to adequate device performance. See Ptak et al., “Effects oftemperature, nitrogen ion, and antimony on wide depletion widthGaInNAs,” Journal of Vacuum Science Technology B 25(3) May/June 2007 pp.955-959. Devices reported in that paper have short circuit currents fartoo low for integration into multijunction solar cells. Nevertheless, itis known that Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) with 0.05≦x≦0.07,0.01≦y≦0.02 and 0.02≦z≦0.06 can be used to produce a lattice-matchedmaterial with a band gap of approximately 1 eV that can providesufficient current for integration into a multijunction solar cell.However, the voltages generated by subcells containing this materialhave not exceeded 0.30 V under 1 sun of illumination. See D. B. Jackrelet al., Journal of Applied Physics 101 (114916) 2007. Thus, atriple-junction solar cell with this material as the bottom subcell hasbeen expected to be only a small improvement upon an analogous triplejunction solar cell with a bottom subcell of Ge, which produces an opencircuit voltage of approximately 0.25 V. See H. Cotal et al., Energy andEnvironmental Science 2 (174) 2009. What is needed is a material that islattice-matched to Ge and GaAs with a band gap near 1 eV that producesan open circuit voltage greater than 0.30 V and sufficient current tomatch (Al)InGaP and (In)GaAs subcells. Such a material would also beadvantageous as a subcell in high efficiency solar cells with 4 or morejunctions.

SUMMARY OF THE INVENTION

According to the invention, an alloy composition is provided that has abandgap of at least 0.9 eV, namely, Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z)with a low antimony (Sb) content and with enhanced indium (In) contentand enhanced nitrogen (N) content as compared with known alloys ofGaInNAsSb, achieving substantial lattice matching to GaAs and Gesubstrates and providing both high short circuit currents and high opencircuit voltages in GaInNAsSb subcells suitable for use in multijunctionsolar cells. The composition ranges forGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) are 0.07≦x≦0.18, 0.025≦y≦0.04 and0.001≦z≦0.03. These composition ranges employ greater fractions of Inand N in GaInNAsSb than previously taught and allow the creation ofsubcells with bandgaps that are design-tunable in the range of 0.9-1.1eV, which is the range of interest for GaInNAsSb subcells. Thiscomposition range alloy will hereinafter be denoted “low-antimony,enhanced indium-and-nitrogen GaInNAsSb” alloy. Subcells of such an alloycan be grown by molecular beam epitaxy (MBE) and should be able to begrown by metallorganic chemical vapor deposition (MOCVD), usingtechniques known to one skilled in the art.

The invention described herein reflects a further refinement of workdescribed in U.S. patent application Ser. No. 12/217,818, including thediscovery and identification of specific ranges of elements, i.e., aspecific alloy mix of the various elements in GaInNAsSb that improvesignificantly the performance of the disclosed solar cells.

The invention will be better understood by reference to the followingdetailed description in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-section of a three junction solar cellincorporating the invention.

FIG. 1B is a schematic cross-section of a four junction solar cellincorporating the invention.

FIG. 2A is a schematic cross-section of a GaInNAsSb subcell according tothe invention.

FIG. 2B is a detailed schematic cross-section illustrating an exampleGaInNAsSb subcell.

FIG. 3 is a graph showing the efficiency versus band gap energy ofsubcells formed from different alloy materials, for comparison.

FIG. 4 is a plot showing the short circuit current (J_(sc)) and opencircuit voltage (V_(oc)) of subcells formed from different alloymaterials, for comparison.

FIG. 5 is a graph showing the photocurrent as a function of voltage fora triple junction solar cell incorporating a subcell according to theinvention, under 1-sun AM1.5D illumination.

FIG. 6 is a graph showing the photocurrent as a function of voltage fora triple junction solar cell incorporating a subcell according to theinvention, under AM1.5D illumination equivalent to 523 suns.

FIG. 7 is a graph of the short circuit current (J_(sc)) and open circuitvoltage (V_(oc)) of low Sb, enhanced In and N GaInNAsSb subcellsdistinguished by the strain imparted to the film by the substrate.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A is a schematic cross-section showing an example of a triplejunction solar cell 10 according to the invention consisting essentiallyof a low Sb, enhanced In and N GaInNAsSb subcell 12 adjacent the Ge,GaAs or otherwise compatible substrate 14 with a top subcell 16 of(Al)InGaP and a middle subcell 18 using (In)GaAs. Tunnel junction 20 isbetween subcells 16 and 18, while tunnel junction 22 is between subcells18 and 12. Each of the subcells 12, 16, 18 comprises several associatedlayers, including front and back surface fields, an emitter and a base.The named subcell material (e.g., (In)GaAs) forms the base layer, andmay or may not form the other layers.

Low Sb, enhanced In and N GaInNAsSb subcells may also be incorporatedinto multifunction solar cells with four or more junctions withoutdeparting from the spirit and scope of the invention. FIG. 1B shows onesuch four-junction solar cell 100 with a specific low Sb, enhanced Inand N GaInNAsSb subcell 12 as the third junction, and with a top subcell16 of (Al)InGaP, a second subcell 18 of (In)GaAs and a bottom subcell140 of Ge, which is also incorporated into a germanium (Ge) substrate.Each of the subcells 16, 18, 12, 140 is separated by respective tunneljunctions 20, 22, 24, and each of the subcells 16, 18, 12, 140 maycomprise several associated layers, including optional front and backsurface fields, an emitter and a base. The named subcell material (e.g.,(In)GaAs) forms the base layer, and may or may not form the otherlayers.

By way of further illustration, FIG. 2A is a schematic cross-section ingreater detail of a GaInNAsSb subcell 12, according to the invention.The low Sb. enhanced In and N GaInNAsSb subcell 12 is thereforecharacterized by its use of low Sb, enhanced In and N GaInNAsSb as thebase layer 220 in the subcell 12. Other components of the GaInNAsSbsubcell 12, including an emitter 26, an optional front surface field 28and back surface field 30, are preferably III-V alloys, including by wayof example GaInNAs(Sb), (In)(Al)GaAs, (Al)InGaP or Ge. The low Sb,enhanced In and N GaInNAsSb base 220 may either be p-type or n-type,with an emitter 26 of the opposite type.

To determine the effect of Sb on enhanced In and N GaInNAsSb subcellperformance, various subcells of the type (12) of the structure shown inFIG. 2B were investigated. FIG. 2B is a representative example of themore general structure 12 in FIG. 2A. Base layers 220 with no Sb, low Sb(0.001≦z≦0.03) and high Sb (0.03≦z≦0.06) were grown by molecular beamepitaxy and were substantially lattice-matched to a GaAs substrate (notshown). These alloy compositions were verified by secondary ion massspectroscopy. The subcells 12 were subjected to a thermal anneal,processed with generally known solar cell processing, and then measuredunder the AM1.5D spectrum (1 sun) below a filter that blocked all lightabove the GaAs band gap. This filter was appropriate because a GaInNAsSbsubcell 12 is typically beneath an (In)GaAs subcell in a multijunctionstack (e.g., FIGS. 1A and 1B), and thus light of higher energies willnot reach the subcell 12.

FIG. 3 shows the efficiencies produced by the subcells 12 grown withdifferent fractions of Sb as a function of their band gaps. The indiumand nitrogen concentrations were each in the 0.07 to 0.18 and 0.025 to0.04 ranges, respectively. It can be seen that the low Sb, enhanced Inand N GaInNAsSb subcells (represented by triangles) have consistentlyhigher subcell efficiencies than the other two candidates (representedby diamonds and squares). This is due to the combination of high voltageand high current capabilities in the low Sb, enhanced In and N GaInNAsSbdevices. (See FIG. 4). As can be seen in FIG. 4, both the low and highconcentration Sb devices have sufficient short-circuit current to matchhigh efficiency (Al)InGaP subcells and (In)GaAs subcells (≧13 mA/cm²under the filtered AM1.5D spectrum), and thus they may be used intypical three-junction or four-junction solar cells 10, 100 withoutreducing the total current through the entire cell. Thiscurrent-matching is essential for high efficiency. The devices withoutSb have relatively high subcell efficiencies due to their high opencircuit voltages, but their short circuit currents are too low for highefficiency multijunction solar cells, as is shown in FIG. 4.

FIG. 4 also confirms that Sb has a deleterious effect on voltage, aspreviously reported for other alloy compositions. However, in contrastto what has been previously reported for other alloy compositions, theaddition of antimony does NOT decrease the short circuit current. Thelow Sb-type subcells have roughly 100 mV higher open-circuit voltagesthan the high Sb-type subcells. To illustrate the effect of thisimprovement, a triple-junction solar cell 10 with an open circuitvoltage of 3.1 V is found to have 3.3% higher relative efficiencycompared to an otherwise identical cell with an open circuit voltage of3.0 V. Thus, the inclusion of Sb in GaInNAs(Sb) solar cells is necessaryto produce sufficient current for a high efficiency solar cell, but onlyby using low Sb (0.1-3%) can both high voltages and high currents beachieved.

Compressive strain improves the open circuit voltage of low Sb, enhancedIn and N GaInNAsSb subcells 10, 100. More specifically, low Sb, enhancedIn and N GaInNAsSb layers 220 that have a lattice constant larger thanthat of a GaAs or Ge substrate when fully relaxed (≦0.5% larger), andare thus under compressive strain when grown pseudomorphically on thosesubstrates. They also give better device performance than layers with asmaller, fully relaxed lattice constant (under tensile strain).

FIG. 7 shows the short circuit current and open circuit voltage of lowSb, enhanced In and N GaInNAsSb subcells grown on GaAs substrates undercompressive strain (triangles) and tensile strain (diamonds). It can beseen that the subcells under compressive strain have consistently higheropen circuit voltages than those under tensile strain.

Low Sb, enhanced In and N, compressively-strained GaInNAsSb subcellshave been successfully integrated into high efficiency multijunctionsolar cells. FIG. 5 shows a current-voltage curve of a triple-junctionsolar cell of the structure in FIG. 1A under AM1.5D illuminationequivalent to 1 sun. The efficiency of this device is 30.5%. FIG. 6shows the current-voltage curve of the triple junction solar celloperated under a concentration equivalent to 523 suns, with anefficiency of 39.2%.

The invention has been explained with reference to specific embodiments.Other embodiments will be evident to those of ordinary skill in the art.It is therefore not intended for the invention to be limited, except asindicated by the appended claims.

1-9. (canceled)
 10. A multijunction solar cell comprising: at least onesubcell comprising Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z), the at least onesubcell having a low antimony (Sb) content and enhanced indium (In)content and enhanced nitrogen (N) content, wherein the content levelsare selected to achieve a bandgap of at least 0.9 eV, wherein thecontent values for x, y, and z are within compositions ranges asfollows: 0.07≦x≦0.18, 0.025≦y≦0.04 and 0.001≦z≦0.03, wherein lowcorresponds to the content value for z and enhanced corresponds to thecontent value for x.
 11. The multijunction solar cell according to claim10, wherein the at least one Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcellis substantially lattice matched to a substrate.
 12. The multijunctionsolar cell according to claim 11, further comprising at least oneadditional subcell that is substantially lattice matched to the at leastone Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcell.
 13. The multijunctionsolar cell according to claim 11, wherein the at least oneGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcell has a lattice constant thatis within 0.5% of the lattice constant of the substrate.
 14. Themultijunction solar cell according to claim 10, wherein the at least oneGa_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcell is capable of producing anopen circuit voltage of at least 0.3 V under illumination of themultijunction solar cell at an intensity of 1,000 W/m² using the AM1.5Dspectrum.
 15. The multijunction solar cell according to claim 10,wherein the at least one Ga_(1-x)In_(x)N_(y)As_(1-y-z)Sb_(z) subcell isnot a current-limiting subcell upon illumination of the multijunctionsolar cell using the AM1.5D spectrum.